Hundreds of thousands of children will fall into poverty in the next few years unless urgent steps are taken to ensure that the poorest are protected from impending budget cuts imposed on local authorities by Whitehall.

The claim is made by End Child Poverty, a coalition of more than 150 charities, welfare organisations, social justice groups and unions that this week publishes a map of UK poverty blackspots.

The map, which uses similar measurements of poverty to those employed by the government, found that in 68 wards in the UK, most children were classed as living in poverty, down from 100 wards the previous year. However, the drop was largely due to falling incomes having skewed the relative poverty threshold, defined as families on or below 60% of median household income.

The campaign predicts that, as benefits start to fall in real terms later this year, the proportion of children living in poverty will increase significantly.

It cites work by the Institute for Fiscal Studies that predicts 400,000 more children will end up in poverty by 2015, rising to 800,000 by 2020.

The campaign's map reveals that the worst affected constituency is Manchester Central, where 47% of children are classed as living in poverty, just ahead of Belfast West and Glasgow North East where the figure is 43%. In Birmingham, Ladywood and Bethnal Green and Bow in London, 42% of children are classed as living in poverty.

The new figures reveal the wide disparity in poverty rates within regions. In London, Poplar and Limehouse has a child poverty rate of 41% compared with 7% in Richmond. In the north west of England, 38% of children in Manchester are poor, compared with 7% in the nearby Ribble Valley.

In the north east the relative decline in child poverty rates is already being reversed, prompting concerns that cuts will exacerbate the north-south divide.

In wards such as Thorntree in Middlesbrough and Grangetown in Redcar and Cleveland, more than 60% of children are classed as living in poverty.

"There are far too many areas of the country where poverty scars the lives of more than a third of children living in the local community," said Enver Solomon, chair of the End Child Poverty coalition and director at the National Children's Bureau. "It is a tragedy that so many children whose parents are struggling to making a living are having to go hungry and miss out on the essentials of a decent childhood that all young people should be entitled to."

The introduction of the "bedroom tax", which will see a reduction in benefits for families with a spare room, and changes to who receives support with paying council tax, will end up costing the most vulnerable more, say welfare groups.

The Resolution Foundation predicts that low-income families will see their council tax rise by up to £600 a year as a result of the changes.

From April, local authorities will be given greater discretion over the allocation of financial support for families. Already, a number of authorities have said they will not be introducing council tax support schemes, claiming they cannot afford them.

But End Child Poverty said it was vital authorities used their new powers to ensure money went to the most vulnerable.

"Local authorities are having to deal with reduced budgets but they must prioritise low-income families in the decisions they make about local welfare spending, including spending on the new council tax benefit, and on protecting families hit by the bedroom tax," Solomon said. "They have a critical role to play in developing effective local strategies to tackling child poverty."

One in five children is currently classed as living below the poverty line – although the definition of what is poor is due to be revised.

The government has been consulting on a new definition of child poverty which includes income but also wider measures to tackle the root causes of poverty including worklessness, educational failure and family breakdown.

A Department for Work and Pensions spokesman said the previous government had set a target of reducing the number of children living in relative income poverty to 1.7m by last year, but the figure had actually risen to 2.3m.

"Despite paying out £171bn in tax credits over the last decade alone, the previous government failed to meet their target to halve child poverty by 2010 and far too many children were left behind," the spokesman explained.

"Our welfare reforms will improve the lives of some of the poorest families in our communities, with the Universal Credit simplifying the complex myriad of benefits and making 3million people better off," the spokesman added.

But End Child Poverty said it feared the changes would have the opposite effect. "Our map shows that in every region the gross disparities in child poverty levels that have existed for many years are deeply entrenched and are set to become an enduring reality as many more children become trapped in long-term poverty and disadvantage," Solomon said.